1. Field of the Invention
The invention relates to detectors for detecting trace amounts of particles of interest.
2. Description of the Related Art
Terrorism risks continue to exist at transportation facilities, government buildings and other high profile locations where there is a significant flow of pedestrian or vehicular traffic. As a result, most airports and many government buildings now include apparatus for detecting trace amounts of explosives. These devices typically operate on the principle that small amounts of the explosive materials will be transferred to the body, clothing and luggage of people who had handled the explosive.
Some detectors employ small flexible fabric-like traps that can be wiped across a package or piece of luggage. The trap removes residue from the surface of the package or luggage. The trap then is placed in an apparatus, such as an ion trap mobility spectrometer, that tests the residue on the trap for trace amounts of explosive materials. A device of this type is disclosed in U.S. Pat. No. 5,491,337 and is marketed by the GE Ion Track. These devices typically are employed in proximity to metal detectors at airports, and security personnel will perform screening on some of the passengers based on a random sampling or based on a determination that the passenger has met certain criteria for enhanced screening.
The ion trap mobility spectrometer disclosed in U.S. Pat. No. 5,491,337 also can operate in a mode for detecting trace amounts of narcotics. Narcotics are illegal and insidious. Furthermore, it is known that many terrorists organizations fund their terrorism through the lucrative sale of narcotics.
Only a fraction of airline passengers have their carry-on baggage checked for trace amounts of explosives or narcotics using fabric-like traps and the available ion trap mobility spectrometers or similar devices. Efforts to use such devices to check all carry-on bags for trace amounts of explosives or narcotics would impose greater time and cost penalties on the airline industry. Additionally, the above-described explosive detectors typically are used only on luggage and other parcels. An apparatus of this type would not identify plastic explosives worn by a passenger who had no carry-on luggage.
U.S. Pat. No. 6,073,499 discloses a walk-through detector. The detector shown in U.S. Pat. No. 6,073,499 operates under the principle that a boundary layer of air adjacent to a person is heated by the person. This heated air adjacent a person is less dense than air farther from the person. Less dense air rises. Accordingly, a thermal plume of air referred to as a human convection plume flows up adjacent to the person at a rate of about 50 liters per second. Minute particles, including particles of explosives or narcotics that may have been handled by the person, will be entrained in this human convection plume of air and will flow up from the person. U.S. Pat. No. 6,708,572 shows a walk-through detector with a plurality of high-pressure air jets that direct small puffs of air towards the torso of the person in the portal. These jets of air help to stimulate a release of particles from the clothing and hands of the person so that a greater concentration of particles can become entrained in the human convection plume.
The above-described walk-through detector has a metallic screen incorporated into the ceiling of the portal. A vacuum pump or fan above the screen generates an airflow that is intended to match the volumetric flow of air generated by the human convection plume (e.g., about 50 liters/second). An airflow generated by the vacuum pump or fan that is too low will permit particles entrained in the human convection plume to dissipate into the ambient air on currents of ambient air near the detector. A flow rate generated by the vacuum pump or fan that is too high will draw additional air through the screen and hence will dilute the concentration of particles of interest. Some of the particles entrained in the thermal plume will attach to the screen. The screen in the ceiling of the portal is moved into a desorber after a sufficient sampling time (e.g., 5 seconds) and is heated to temperatures in the range of 220° C.–250° C. so that particles thereon are vaporized. The vaporized particles then are drawn into the inlet of the ion mobility spectrometer, the ion trap mobility spectrometer or other such detecting device to determine whether any particles of interest were entrained in the human convection plume. An alarm or other signal will be triggered if a particle of interest is detected. The above-described walk-through detector typically operates at an overall average sampling efficiency of about 1% at mid-torso level. This level of efficiency typically meets government standards for detection and offers low false alarm rates. Thus, the walk-through detector disclosed in U.S. Pat. No. 6,073,499 and in U.S. Pat. No. 6,708,572 is very effective for detecting whether a person is carrying explosives or narcotics and whether the person has recently handled explosives or narcotics. However, improved efficiencies would be received well and could be even more effective for detecting even smaller amounts of particles of interest without increasing the false alarm rate.
The walk-through detector disclosed in U.S. Pat. No. 6,073,499 and U.S. Pat. No. 6,708,572 is marketed by GE Ion Track as the EntryScan3® and currently operates at about a fifteen second cycle to sample, desorb and analyze a passenger. The person being screened must pause in the portal of the walk-through detector for at least the sampling phase of that cycle, and typically at least about 5 seconds. A system that achieves a shorter cycle time would be well received in the industry.
Inertial impactors use principles enunciated in Stokes Law and function to collect particles entrained in a gas. Such impactors have been used, for example, in the analysis of air quality and are shown, for example, in U.S. Pat. No. 6,435,043 and U.S. Pat. No. 6,732,569. Inertial impactors have not been used to identify extremely small particles (e.g., 1 micron) of explosives or narcotics that may exist at low concentration in an air stream of 50–100 liters per second.
In view of the above, it is an object of the subject invention to provide a detector with improved sample collection efficiency.
Another object of the invention is to provide a detector with a shorter cycle time from sample collection to analysis, and hence a detector with a higher passenger throughput.
A further object of the invention is to provide a detector with fewer or no moving parts.